Production method for simox substrate and simox substrate

ABSTRACT

The present invention provides a SIMOX substrate produced by employing an oxygen ion implantation amount in a low dose range, which substrate is a high quality SOI substrate having an increased thickness of a BOX layer, and a method of producing the same, and more specifically, provides a method of producing a SIMOX substrate wherein a buried oxide layer and a surface silicon layer are formed by applying the implantation of oxygen ions in a silicon substrate and a high temperature heat treatment thereafter, characterized by: forming the buried oxide layer through applying a high temperature heat treatment after an oxygen ion implantation; then applying an additional oxygen ion implantation so that the peak position of the distribution of implanted oxygen is located at a portion lower than the interface between the buried oxide layer, already formed, and the substrate thereunder; and then applying another high temperature heat treatment, and a SIMOX substrate produced by said method having a surface silicon layer 10 to 400 nm in thickness and a buried oxide layer 60 to 250 nm in thickness.

TECHNICAL FIELD

[0001] The present invention relates to an SOI substrate having a buriedoxide layer near a surface of a silicon substrate and a single crystalsilicon surface layer (hereinafter called an SOI layer, meaning asilicon-on-insulator layer) formed thereon.

[0002] 2. Background Art

[0003] A Separation by IMplanted OXygen (SIMOX) wafer and a bonded waferare known as typical examples of an SOI substrate which has a singlecrystal silicon layer formed on an insulating material such as silicondioxide. The SIMOX wafer is an SOI substrate obtained by: introducingoxygen ions inside a single crystal silicon substrate through the ionimplantation of oxygen ions; having the oxygen ions react with siliconatoms through a subsequent annealing process; and thus forming a BuriedOXide layer (hereinafter called a BOX layer). The bonded wafer, on theother hand, is an SOI substrate obtained by: bonding two single crystalsilicon wafers, with an oxide layer in between; and then turning one ofthe two wafers into a thin film.

[0004] A Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET)formed in the SOI layer of such an SOI substrate has high radiationresistance and high latch-up resistance, shows high reliability,suppresses a short channel effect which tends to occur as a device sizebecomes smaller, and enables an operation with low power consumption, aswell. Besides, because the operating region of a device is isolated fromthe substrate itself in terms of dielectric capacitance, signaltransmission speed is enhanced, and thus high-speed operation of thedevice is realized. For those reasons, the SOI substrate has been themost promising canditate of a high performance semiconductor substratefor MOS-LSIs of the next generation.

[0005] Among the variety of the SOI substrates, the SIMOX wafer has anadvantage in its excellent uniformity of the thickness of the SOI layer.In the SIMOX wafer, it is possible to form an SOI layer 0.4 μm or lessin thickness and even to control the thickness of the SOI layer to 0.1μm, or even smaller. An SOI layer 0.1 μm or less in thickness, inparticular, is often used for forming a MOS-LSI for fully depletion typeoperation. In this case, as the thickness of the SOI layer itself is ina proportional relationship with the threshold voltage of a MOSFET, theuniformity of the thickness of the SOI layer constitutes an importantquality aspect for producing devices of uniform performance at a highyield. In this regard, the SIMOX wafer having an SOI layer excellent inuniformity of the thickness has been the most promising canditate of thesubstrates for MOSFETs of the next generation.

[0006] A MOS-LSI formed on an SOI substrate is capable of realizing theexcellent properties described above, such as the enhanced radiationresistance and latch-up resistance, the operation with low powerconsumption, the ultra-high-speed operation, etc., thanks to the factthat the device active area is dielectrically isolated from thesubstrate itself by laying the BOX layer which is an insulator betweenthem. In this respect, the BOX layer is required to have as perfect aninsulating property as possible. More concretely, it is required thatthe leak defects in the BOX layer (hereinafter called pinhole defects)are as small as possible and the dielectric breakdown property is asclose to that of a thermally-grown oxide layer as possible.

[0007] In the production of a SIMOX wafer, oxygen ions are implanted byapplying, usually, a fixed amount of accelerating energy, typically, anaccelerating voltage of 200 kV or so. It is widely known that, in theSIMOX structure formed after a high temperature heat treatment, acontinuous, uniform and high quality BOX layer can be obtained only whenthe implantation dosing amount of the oxygen ions is 1.5×10¹⁸ cm⁻² ormore, or within a limited range from 2.5 to 4.5×10¹⁷ cm⁻² (refer to S.Nakashima and K. Izumi, the Journal of Materials Research, Vol. 8, p.523(1993), for example). The SIMOX wafer produced by implanting the oxygenion amount specified above is conventionally called: a high dose SIMOXsubstrate when produced by implanting the oxygen ion amount in theformer range; or a low dose SIMOX substrate when produced by implantingthe oxygen ion amount in the latter range.

[0008] The high dose SIMOX substrate and the low dose SIMOX substratehave respective characteristics, and they are used for differentapplications according to the characteristics. Out of the two types ofSIMOX substrates, in the case of the low dose SIMOX substrate, thedosing amount of oxygen ion implantation is comparatively small,therefore the threading dislocation density of the SOI layer isdecreased, and the low dose SIMOX substrate is expected to be atechnology capable of realizing low-cost production. The low dose SIMOXsubstrate, however, has problems such as the relatively large density ofpinhole defects in the BOX layer, the high probability of dielectricbreakdown property of the BOX layer becoming insufficient and so forth,because the BOX layer is thin. With regard to the BOX quality of the lowdose SIMOX substrate, when it is attempted to increase the thickness ofthe BOX layer by simply increasing the oxygen ion implantation amount,although the pinhole defects decrease, granular substances consisted ofsilicon (hereinafter called silicon islands) are generated in quantitiesinside the BOX layer and, as a result, the breakdown electric field ofthe BOX layer is lowered. However, it has been pointed out that, whenthe oxygen ion implantation dosing amount is decreased, in contrast,though the silicon islands decrease and the breakdown electric field ofthe BOX layer is improved, the density of the pinhole defects increasesas the oxygen ion implantation dosing amount decreases. It has thereforebeen very difficult to simultaneously improve the whole quality of theBOX layer in the low dose SIMOX substrate produced according to aconventional technology.

[0009] As a technology to contribute to the quality improvement of theBOX layer of the low dose SIMOX substrate, a technology has beenproposed which makes use of internal oxidation at a high temperature(internal thermal oxidation process, hereinafter called the ITOXtechnology in abbreviation) (Nakashima et al.: Japanese UnexaminedPatent Publication No. H07-263538, or S. Nakashima et al.: the Journalof Electrochemical Society, Vol. 143, p.244). According to the ITOXtechnology, a thermally-grown oxide layer grows on the substrate surfacethrough an oxidation treatment at a high temperature, and, at the sametime, some amount of thermally-grown oxide layer at the upper interfaceof the BOX layer, and thus it becomes possible to make the BOX layerthicker. Further, it has been reported that the pinhole defects aredecreased as a result, and the breakdown electric field is improved atthe same time. In the ITOX technology, however, it is necessary for thesurface oxide layer to grow by not less than 10 times the increase inthe thickness of the BOX layer. For this reason, to secure a prescribedthickness of the SOI layer in the SIMOX structure finally obtained, itis necessary to restrict the amount of oxidation of the substratesurface. As a consequence, there has been a natural limit in theincrement of the BOX layer thickness.

[0010] As a measure to make the BOX layer of a SIMOX substrate thickerwithout being influenced by the above restriction, it has been proposedthat, in a method wherein a series of oxygen ion implantations isconducted while changing the average depth of the implantationstep-by-step or continuously and a heat treatment at a high temperatureis applied thereafter, the cumulative distribution of implanted oxygenis controlled so that it falls, after the series of oxygen ionimplantation, within a range in which the silicon islands are notgenerated, and that the distribution has a single peak so as to form asingle BOX layer after the high temperature heat treatment (see JapaneseUnexamined Patent Publication No. H7-201975). By this technique, it istheoretically possible to obtain a high quality BOX layer even underdosing conditions outside the so-called dose window, under which dosingconditions it is impossible to obtain a good quality BOX layer when theoxygen ion is implanted using a fixed accelerating voltage. However, itis necessary, in the proposed technique, to control the peak value ofthe distribution of implanted oxygen to not more than 2.25×10²² cm⁻³,i.e. about a half of 4.48×10²² cm⁻³, which is the oxygen concentrationin silicon dioxides, in order to prevent the silicon islands fromforming in the BOX layer, and, if a plurality of peaks appear in thedistribution of implanted oxygen after the series of oxygen ionimplantation caused by changes in process conditions or other factors,precipitation takes place during the high temperature heat treatmentaround each of the peaks and a single BOX layer is not formed. Becauseof the unstable nature of the proposed method as described above, it hasa problem that precise and delicate control of the process is requiredin order to avoid the instability.

[0011] In the meantime, as a technology wherein oxygen ion implantationand high temperature heat treatment are repeated, a technology has beenproposed wherein oxygen ion implantation at a fixed accelerating energyand high temperature heat treatment are repeated for the purpose ofimproving the quality of a high dose SIMOX substrate (see JapaneseUnexamined Patent Publication No. H1-17444). The object of the proposedtechnology is to reduce defects, such as threading dislocations, in theSOI layer finally obtained by reducing the damage generated in thesubstrate at each step of the implantation through dividing theimplantation of a required dosing amount into a plurality of steps andrecovering further from the reduced damage through applying a hightemperature heat treatment after each of the implantation steps. In thistechnique, however, the implantation of the oxygen ion amountconventionally employed for producing a high dose SIMOX substrate isdivided into several steps, and it causes no change in the thickness ofthe BOX layer finally obtained. Besides, it has been pointed out that,using the technique, waviness occurs conspicuously in the BOX layerformed at an intermediary step and, as a consequence, the flatness ofthe interface of the BOX layer finally obtained is deteriorated.

[0012] As a measure to avoid the problem, a proposal has been made,wherein the implantation is divided into two steps and a loweraccelerating energy than that in the first step implantation is appliedin the second step implantation, and by so doing, oxygen ions areimplanted preponderantly at the interface of the BOX layer formed afterthe first step heat treatment and the SOI layer, and thus the flatnessof the interface is improved (see Japanese Unexamined Patent PublicationNo. H4-249323). However, this technology is also an invention aiming atimproving the quality of the high dose SIMOX substrate and, as theimplantation of the total oxygen ion amount conventionally employed forproducing a high dose SIMOX substrate is divided into steps, like in theprevious example, it causes no change in the thickness of the BOX layerfinally obtained. What is more, because the oxygen ions are implanted inthe second step on the upper side of the BOX layer already formed in thefirst step, the SOI layer finally obtained tends to be thin. Theproposed technology is, therefore, not suitable for increasing thethickness of the BOX layer while maintaining the thickness of the SOIlayer.

[0013] The object of the present invention is to provide a technologycapable of increasing the thickness of the buried oxide layer of theSIMOX substrate without the above difficulties. In particular, theobject of the present invention is to provide a high quality SOIsubstrate for a high performance LSI by making it possible to increasethe thickness of the BOX layer of a SIMOX substrate produced byemploying the oxygen ion implantation amount in the low dose range,keeping good quality of the BOX layer, and a method of producing the SOIsubstrate.

DISCLOSURE OF THE INVENTION

[0014] In the SIMOX method wherein an SOI structure is formed byapplying the implantation of oxygen ions into a single crystal siliconsubstrate and a heat treatment at a high temperature thereafter, thepresent inventors newly discovered that it is possible to increase thethickness of the BOX layer, while maintaining the thickness andcrystallinity of the SOI layer and suppressing the formation of thesilicon islands in the BOX layer, if, after a BOX layer is once formedthrough a high temperature heat treatment, oxygen ions are implantedagain in a position deeper than the BOX layer thus formed and then aheat treatment at a high temperature is applied once again. The presentinvention, therefore, relates to a method of producing an SOI substratewhich solves the above problems and, further, to the SOI substrateproduced by applying the technologies of the method, and it is realizedby the following means.

[0015] A method of producing a SIMOX substrate wherein a BOX layer andan SOI layer are formed by applying the implantation of oxygen ions intoa silicon substrate and a heat treatment at a high temperaturethereafter, characterized by: forming a BOX layer through applying ahigh temperature heat treatment after an oxygen ion implantation; andthen, repeatedly, applying oxygen ion implantation so that the peakposition of the distribution of implanted oxygen is located at a portionlower than the interface between the BOX layer, already formed, and thesubstrate thereunder, and a heat treatment at a high temperaturethereafter.

[0016] Further, a method of producing a SIMOX substrate according to theabove, characterized in that the dosing amount of an additional oxygenion implantation does not exceed the total dosing amount of the oxygenions implanted earlier.

[0017] Further, a method of producing a SIMOX substrate according to theabove, characterized in that the accelerating energy applied to theoxygen ion implantation is different from that applied to an additionaloxygen ion implantation.

[0018] Further, a method of producing a SIMOX substrate according to theabove, characterized by partially removing the surface of the SOI layeralready formed before the additional oxygen ion implantation.

[0019] Further, a method of producing a SIMOX substrate according to theabove, characterized in that the method of removing the surface of theSOI layer is etching using a reactive substance.

[0020] Otherwise, a method of producing a SIMOX substrate according tothe above, characterized in that the method of removing the surface ofthe SOI layer is the removal of an oxide layer after forming it byoxidizing the surface of the substrate.

[0021] Otherwise, a method of producing a SIMOX substrate according tothe above, characterized in that the method of removing the surface ofthe SOI layer is surface polishing.

[0022] Additionally, in the method of producing a SIMOX substrateaccording to the above, it is desirable that the number of repetitionsof the oxygen ion implantation and the high temperature heat treatmentis two.

[0023] Further, in any of the methods of producing a SIMOX substrateaccording to the above, it is desirable to form an oxide layer on thesurface of the silicon substrate prior to the oxygen ion implantationand to remove the oxide layer after the oxygen ion implantation or thehigh temperature heat treatment.

[0024] Further, in the method of producing a SIMOX substrate accordingto the above, it is desirable that the oxygen ion implantation isconducted under a condition of an accelerating energy of 150 to 250 keVand a dosing amount of 2 to 6×10¹⁷ cm⁻², that an additional oxygen ionimplantation is conducted under a condition of an accelerating energy of150 to 250 keV and a dosing amount of 0.1 to 6×10¹⁷ cm⁻², and that thetotal depth of the removal of the silicon surface is 20 to 300 nm.

[0025] The present invention is, further, a SIMOX substrate produced byapplying any of the methods according to the above, characterized inthat the thickness of the SOI layer of the SIMOX substrate is 10 to 400nm and the thickness of the BOX layer thereof is 60 to 250 nm.

[0026] According to the present invention, a BOX layer similar to thatof a conventional low dose SIMOX substrate is once formed through anoxygen ion implantation and a high temperature heat treatment. For thisreason, when the oxygen implanted at an additional oxygen ionimplantation precipitates during the subsequent high temperature heattreatment, the precipitation takes place stably, because the BOX layeralready formed exists and the precipitating oxygen is absorbed in theBOX layer. As a result, it is possible to obtain a single BOX layerstably. In addition, by the present invention, it is also possible toincrease the thickness of the BOX layer and, at the same time, reducethe density of the pinhole defects in the BOX layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 shows the production processes 1) and 2) of a SIMOXsubstrate according to a conventional technology and schematic sectionalillustrations of silicon substrates at the production processes and thedistributions of oxygen concentration in the silicon substrates.

[0028]FIG. 2 shows the production processes 3) and 4) of a SIMOXsubstrate according to the first embodiment of the present invention andschematic sectional illustrations of silicon substrates at theproduction processes and the distributions of oxygen concentration inthe silicon substrates.

[0029]FIG. 3 shows the production processes 3), 4) and 5) of a SIMOXsubstrate according to the second embodiment of the present inventionand schematic sectional illustrations of silicon substrates at theproduction processes and the distributions of oxygen concentration inthe silicon substrates.

[0030]FIG. 4 shows the production processes 3), 4), 5) and 6) of a SIMOXsubstrate according to the third embodiment of the present invention andschematic sectional illustrations of silicon substrates at theproduction processes and the distributions of oxygen concentration inthe silicon substrates.

[0031]FIG. 5 is a graph showing the density of the pinhole defects inthe BOX layer, comparing the first to third examples of the presentinvention, a conventional example and a comparative example.

[0032]FIG. 6 is a graph showing the breakdown electric field of the BOXlayer, comparing the first to third examples of the present invention, aconventional example and a comparative example.

[0033]FIG. 7 is a graph showing the density of defects in the SOI layer,comparing the first to third examples of the present invention, aconventional example and a comparative example.

BEST MODE FOR CARRYING OUT THE INVENTION

[0034] Preferable embodiments of the present invention are explainedhereafter based on the schematic sectional illustrations in FIGS. 1 to4.

[0035] The first half of the processes to produce a SIMOX substrateaccording to an embodiment of the present invention employs theproduction processes of the SIMOX substrate according to a conventionaltechnology shown in FIG. 1. In the processes shown in FIG. 1, 1) anoxygen implanted zone 2′ is formed by charging a single crystal siliconsubstrate 1 into an ion implantation apparatus and implanting oxygenions up to a prescribed dosing amount in the surface of the substrate,while heating the substrate so as to maintain its crystallinity. Then,the substrate is discharged from the ion implantation apparatus and, 2)a SIMOX structure having an SOI layer 3′ and a BOX layer 4′ is formed byapplying a high temperature heat treatment in a heat treatment furnace.It has to be noted that, when forming a SIMOX substrate in a low doserange by applying, for instance, an accelerating voltage of 150 to 250kV for the oxygen ion implantation, it is desirable, to obtain a highquality BOX layer, to apply a dosing amount of 2 to 6×10¹⁷ cm⁻² as theprescribed dosing amount. The oxygen ion implantation and the hightemperature heat treatment described above are called the first oxygenion implantation and the first high temperature heat treatment,respectively, in the explanations of this description.

[0036] In the first embodiment of the present invention shown in FIG. 2,after completing the processes shown in FIG. 1, the substrate is chargedinto the ion implantation apparatus again, and 3) an oxygen ionimplanted zone 2 is formed by applying oxygen ion implantation undersuch an accelerating voltage that the peak of the distribution of theimplanted oxygen ions is located at the lower portion of the alreadyformed BOX layer 4′. After that, the substrate is discharged from theion implantation apparatus and, then, 4) a SIMOX structure having an SOIlayer 3 and a BOX layer 4 is formed by applying a high temperature heattreatment in a heat treatment furnace. The oxygen ion implantation andthe high temperature heat treatment described above are called thesecond oxygen ion implantation and the second high temperature heattreatment, respectively, in the explanations of this description.

[0037] By the method shown in FIG. 2, the oxygen ions implanted at thesecond oxygen ion implantation aggregate onto the already formed BOXlayer 4′ from the lower side during the subsequent second hightemperature heat treatment. In order to realize the stable aggregationof the oxygen ions in the above process, it is desirable to control thedosing amount of the oxygen ions of the second oxygen ion implantationso as not to exceed that of the first oxygen ion implantation. By sodoing, the precipitation of the oxygen ions proceeds during the secondhigh temperature heat treatment so that the oxygen ions implanted at thesecond oxygen ion implantation aggregate in priority to the BOX layer 4′already formed, without precipitating them independently. What is more,as the precipitation process from the second oxygen ion implantation tothe second high temperature heat treatment hardly affects the upperinterface of the BOX layer 4′, as a result, the BOX layer 4 is madethicker, maintaining good uniformity of the thickness of the SOI layer.

[0038] As is well known, the damage inflicted on the material whereinions are implanted by the implanted ions becomes larger as the ions losetheir energy and decelerate in the material wherein ions are implanted.The degree of the damage inflicted on the material wherein ions areimplanted by the implanted ions, therefore, becomes large at thevicinity immediately above the position where the ions halt. By themethod shown in FIG. 2, it is possible to concentrate the damagegenerated in a substrate by the second ion implantation on the inside orlower portion of the BOX layer 4′ formed during the first hightemperature heat treatment. For this reason, it is possible to suppressthe new damage to the already formed SOI layer 3′ generated by thesecond oxygen ion implantation and, as a result, to suppress thegeneration of defects such as threading dislocations in the SOI layer 3formed after the second high temperature heat treatment.

[0039] In the second embodiment of the present invention shown in FIG.3, after completing the processes shown in FIG. 1, 3) the SOI layer 3′is made thinner by removing a part of its surface. Then, the substrateis charged into the ion implantation apparatus again and 4) oxygen ionimplantation is carried out by applying such an accelerating voltagethat the peak of the distribution of implanted oxygen ions is located atthe lower portion of the already formed BOX layer 4′ as shown in FIG. 2.Thereafter, the substrate is discharged from the ion implantationapparatus and, then, a SIMOX structure having an SOI layer 3 and a BOXlayer 4 is formed by applying a high temperature heat treatment in aheat treatment furnace.

[0040] By the method shown in FIG. 3, it is possible to implant oxygenions in a desired position without changing the accelerating voltageused in the second oxygen ion implantation so much from the acceleratingvoltage used in the first oxygen ion implantation, comparing with themethod shown in FIG. 2. As a consequence, the freedom in the combinationof the accelerating voltage used in the second oxygen ion implantationexpands within the capacity range of the ion implantation apparatusemployed and, for this reason, it becomes possible to expand the freedomin the thicknesses of the SOI and BOX layers finally formed. At the sametime, the load imposed on the ion implantation apparatus by the changeof operating conditions is alleviated. Note that techniques such asreactive ion etching, etching using a mixed solution of hydrofluoricacid and nitrous acid, mechanical polishing, etc. can be used as themethod of removing a part of the SOI layer surface before applying thesecond oxygen ion implantation.

[0041] In the third embodiment of the present invention shown in FIG. 4,after completing the processes shown in FIG. 1, 3) a thermally-grownoxide layer 5 is formed on the surface of the SOI layer 3′, and then 4)the oxide layer is removed. A solvent to selectively dissolve the oxidelayer or the like may be used for the removal. Subsequently, a SIMOXstructure having an SOI layer 3 and a BOX layer 4 is formed by applying5) an ion implantation in the same manner as in FIG. 2 and 6) a heattreatment. By this method, too, the same effects as those explained inthe description of the second embodiment are expected to show.

[0042] It has to be noted that, in the first to third embodiments of thepresent invention shown in FIGS. 2 to 4, respectively, it is acceptableto form an oxide layer on the surface of a silicon substrate prior tothe first oxygen ion implantation and to remove the oxide layer using asolvent as described above after the first oxygen ion implantation orthe first high temperature heat treatment immediately thereafter. Inorder that the oxide layer formed on the substrate surface is not erasedby the sputtering during the oxygen ion implantation in the above case,it is desirable to form the oxide layer in a thickness of 30 nm or more.In order to obtain the SOI layer after the high temperature heattreatment, on the other hand, it is necessary to restrict the thicknessof the surface oxide layer to 400 nm or so at the largest.

[0043] The first to third embodiments of the present invention shown inFIGS. 2 to 4, respectively, are examples wherein the oxygen ionimplantation is repeated twice in total, but the partial removal of theSOI layer and the subsequent oxygen ion implantation and hightemperature heat treatment may be repeated as long as the SOI layer doesnot disappear. When an accelerating voltage of 150 to 250 kV and adosing amount of 2 to 6×10¹⁷ cm⁻² are applied to the oxygen ionimplantation, as explained earlier, it is desirable that the totalremoval depth of the surface of the SOI layer or the surface of thesilicon single crystal is 20 nm or more in order to obtain the effectsof the present invention, and 300 nm or less in order to prevent the SOIlayer from disappearing. In this case, it is possible to form thefinally obtained SOI layer in any desired thickness up to an upper limitof 400 nm or so, as long as it is within a range not letting the SOIlayer disappear, by appropriately controlling process conditions.

[0044] In the first to third embodiments of the present invention shownin FIGS. 2 to 4, respectively, the quality of the BOX layer is alsoimproved with respect to the pinhole defects, the breakdown electricfield and so forth, as the thickness of the BOX layer increases. Theextent of the quality improvement becomes greater as the total of thedosing amounts at the second and subsequent oxygen ion implantationprocesses increases and, to obtain tangible improvement effects withrespect to these quality aspects of the BOX layer, it is desirable thatthe total of the dosing amounts at the second and subsequent oxygen ionimplantation processes is at least 0.1×10¹⁷ cm⁻² or more, morepreferably, 0.5×10¹⁷ cm⁻² or more.

[0045] With regard to an apparatus used for applying the oxygen ionimplantation according to the present invention, any apparatus isacceptable as long as it is capable of accelerating oxygen ions byapplying a voltage and thus implanting them into a silicon wafer throughthe surface, and there are no specific restrictions as to its type, ionimplanting method, etc.

[0046] With regard to the conditions of the first oxygen ionimplantation in the production of the SIMOX substrate according to thepresent invention, the conditions in the low dose range are alreadyshown. However, the conditions of the first oxygen ion implantation arenot necessarily limited to those of the low dose range, and theconditions of the so-called middle dose or high dose range where thedosing amount is larger than those shown above may be applied. For thepurpose of improving the BOX layer quality, such as its thickness,pinhole defects, breakdown electric field, and so forth, however,greater effects are expected when the present invention is applied tothe cases where the conditions of the low dose range are used and muchimprovement in the quality is required. It is also acceptable to divideeach of the first oxygen ion implantation and the second and subsequentoxygen ion implantation into two or more steps. For the purpose ofreducing the crystal defects in the SOI layer, it is desirable that thetemperature of the substrate at least during the first oxygen ionimplantation is 500 to 600° C. or so.

[0047] With regard to an apparatus used for a high temperature heattreatment, there is no specific restriction as long as the apparatus iscapable of carrying out a heat treatment at a desired temperature for adesired period of time. Typically, a high temperature heat treatmentfurnace can be named as a desirable apparatus for the purpose. A lampanneal furnace can also be applied, as long as it satisfies requiredperformances in terms of treatment temperature, time, etc. Theconditions of a heat treatment furnace other than the treatmenttemperature and time, such as charging temperature, heating rate,cooling rate, and so forth are not limited specifically, and the heatingor cooling may be conducted in two or more steps.

[0048] With regard to the conditions of a high temperature heattreatment, it is desirable, for removing the damage caused by the ionimplantation and obtaining a high quality SOI structure, to conduct theheat treatment at a temperature of 1,300° C. or higher but below themelting point of silicon. However, the heat treatment temperature is notlimited to the above temperature range. A non-oxidizing atmosphere usingan inert gas or an atmosphere containing a small amount of oxygen addedto the non-oxidizing atmosphere is desirable as the atmosphere of theheat treatment for removing the damage. However, it is not necessarilylimited to those atmospheres, and an oxidizing atmosphere is alsoacceptable. Argon, nitrogen, and the like are used, typically, as theinert gas. However, it is not necessarily limited to them. Further, ahigh temperature oxidizing treatment may be applied subsequent to thehigh temperature heat treatment.

EXAMPLE

[0049] Examples of the present invention are explained hereafter basedon the schematic sectional illustrations in FIGS. 1 to 4.

[0050] Oxygen ions were implanted in the surface of p-type (100) siliconsubstrates at an accelerating voltage of 180 kV and up to animplantation amount of 4×10¹⁷ cm⁻², according to the processes shown inFIG. 1. The temperature of the substrates during the ion implantationwas set at 600° C. for the purpose of maintaining their crystallinity.Then, the substrates were discharged from the ion implantation apparatusand subjected to a heat treatment at a temperature of 1,300° C. orhigher for 6 h. in a heat treatment furnace to form the SIMOX structure.The heat treatment was conducted in an atmosphere of argon mixed with 1%of or less of oxygen. The structure of the substrate discharged from thefurnace was evaluated by spectroscopic-ellipsometry and, as a result,the SOI layer measured roughly 340 nm in thickness, and the BOX layerwas 85 nm thick.

[0051] In the first example according to the first embodiment of thepresent invention shown in FIG. 2, after completing the processes shownin FIG. 1, the substrate was charged into the ion implantation apparatusonce again and subjected to another oxygen ion implantation process atan accelerating voltage of 210 kV and up to an implantation amount of2×10¹⁷ cm⁻². The temperature of the substrate during the ionimplantation was set, like in the first implantation, at 600° C. Then,the substrate was discharged from the ion implantation apparatus andsubjected to a heat treatment process at a temperature of 1,300° C. orhigher for 6 h. in an atmosphere of argon mixed with 1% of or less ofoxygen in the heat treatment furnace. After the heat treatment, thesubstrate discharged from the furnace was evaluated, as describedearlier, by spectroscopic-ellipsometry. As a result, the SOI layermeasured roughly 280 nm in thickness, and the BOX layer 130 nm.

[0052] In the second example according to the second embodiment of thepresent invention shown in FIG. 3, after completing the processes shownin FIG. 1, the surface of an SOI layer was etched with a mixed solutionof hydrofluoric acid and nitrous acid up to a depth of about 120 nm toobtain an SOI layer roughly 200 nm in thickness. Then, the substrate wascharged into the ion implantation apparatus once again and subjected toan oxygen ion implantation process at an accelerating voltage of 190 kVand up to an implantation amount of 2×10¹⁷ cm⁻². The temperature of thesubstrate during the ion implantation was set, as in the firstimplantation, at 600° C. Then, the substrate was discharged from the ionimplantation apparatus and subjected to a heat treatment at atemperature of 1,300° C. or higher for 6 h. in an atmosphere of argonmixed with 1% of or less of oxygen in the heat treatment furnace. Afterthe heat treatment, the substrate discharged from the furnace wasevaluated, as described earlier, by spectroscopic-ellipsometry. As aresult, the SOI layer measured roughly 160 nm in thickness, and the BOXlayer 130 nm.

[0053] In the third example according to the third embodiment of thepresent invention shown in FIG. 4, after completing the processes shownin FIG. 1, a thermally-grown oxide layer roughly 270 nm in thickness wasformed on a surface of an SOI layer at a temperature of 1,000° C. orhigher and, then, the oxide layer was removed using a solution ofhydrofluoric acid. As a result, a part of the SOI layer about 220 nm inthickness remained on the substrate. The ion implantation and heattreatment were conducted subsequently under the same conditions as inthe second example. The SOI layer and BOX layer finally obtained wereabout 160 nm and 130 nm in thickness, respectively.

[0054] The quality of the BOX layers of the samples thus obtained wasevaluated and compared. Besides the samples of the above three examples,also evaluated were a sample of a conventional example produced throughthe processes shown in FIG. 1 and a sample of a comparative exampleproduced through the processes shown in FIG. 1 such that the oxygen ionimplantation amount was 6×10¹⁷ cm⁻², which was the same amount as thetotal implantation amount of the examples according to the presentinvention.

[0055] The pinhole defects in the BOX layer were evaluated by the copperplating method, wherein: each of the samples was arranged so that onlythe top surface of the substrate touched an electrolyte containingcopper ions and the bottom surface of the substrate touched a cathode,and an anode was placed in the electrolyte; then, copper plated portionswere made to form on the top surface of the substrate immediately abovethe portions where pinholes existed in the BOX layer, by applying to theanode and cathode a low voltage of about 10 V which would not destroythe BOX layer; and the density of the pinholes in the BOX layers wasevaluated by counting the number of the copper plated portions. FIG. 5shows the pinhole density in each of the samples of the conventionalexample, the first to third examples of the present invention and thecomparative example. It was made clear that, in the samples of the firstto third examples and the comparative example, the pinhole density wasreduced to about one fifth that of the sample of the conventionalexample, and that the number of pinholes decreased as the total amountof the oxygen implantation increased.

[0056] In succession, the breakdown electric field of the BOX layer ofeach sample was evaluated, wherein: the SOI layer of each sample wasdivided into sections 1 mm² each by lithography and etching and an Alelectrode was formed on the top surface of each of the sections byvacuum deposition; an Au film was formed on the bottom surface of eachsample substrate as the other electrode by vacuum deposition; and thebreakdown electric field of the BOX layer was evaluated by applying avoltage to the Al electrodes and the Au electrode on the bottom surfaceof each substrate, thus imposing an electric field on the BOX layer, andmeasuring the current flowing at the time. Here, the electric fieldstrength was calculated by dividing the voltage imposed on bothelectrodes by the thickness of the BOX layer.

[0057]FIG. 6 shows the breakdown electric field of the BOX layer in eachof the samples of the conventional example, the first to third examplesand the comparative example. It is clear in the figure that, in thesamples of the first to third examples, the breakdown electric field ofthe BOX layer is higher than that in the conventional example. It wasfound out, on the other hand, that, in the sample of the comparativeexample produced with the same dosing amount as in the first to thirdexamples, the dielectric breakdown characteristic of the BOX layer wasinferior to that of the sample of the conventional example. This ispresumably because the number of the silicon islands in the BOX layerwas increased by simply increasing the dosing amount of the oxygen ionsand, as a result, the breakdown electric field decreased, contrary toexpectation.

[0058]FIG. 7 shows the density of defects evaluated in terms of etch pitin the SOI layer of each sample. It is clear from the figure that, inthe samples of the first to third examples, the defect density is keptat the same level as in the conventional example, despite the fact thatthey underwent additional ion implantation. The figure also shows that,in the sample of the comparative example, to which the same amount ofoxygen ions as in the samples of the first to third examples wasimplanted in one step, in contrast, the defect density of the SOI layeris higher by more than 10%, which indicates that the damage increases asthe amount of the implanted ions increases. This result can beinterpreted as follows: in the first to third examples, the oxygen ionswere implanted at the second oxygen ion implantation in the lower sideof the BOX layer formed through the first heat treatment and, for thisreason, most of the damage generated at the second implantation wasintroduced to the inside or the lower side of the BOX layer and, as aconsequence, the influence of defect formation on the finally formed SOIlayer was alleviated.

[0059] Summarizing the above results, it is clear that, in the first tothird examples according to the present invention, the increase of thedefects in the SOI layer is suppressed in spite of the increase in theoxygen ion implantation amount, while the thickness of the BOX layer isincreased, the number of pinholes in the BOX layer is decreased and thebreakdown electric field is raised, compared with the conventionalexample.

Industrial Applicability

[0060] As is explained above, the present invention makes it possible,in the production of a SIMOX substrate, to obtain a high quality BOXlayer whose thickness is heavier than that of a SIMOX substrate producedby a conventional method, by applying to a SIMOX substrate produced by aconventional method additional oxygen implantation so that the peakposition of the distribution of implanted oxygen is located at theportion lower than the interface between the BOX layer already formedand the substrate thereunder and, then, a high temperature heattreatment.

1. A method of producing a SIMOX substrate wherein a buried oxide layerand a surface silicon layer are formed by applying the implantation ofoxygen ions in a silicon substrate and a high temperature heat treatmentthereafter, characterized by: forming the buried oxide layer throughapplying a high temperature heat treatment after an oxygen ionimplantation; then applying an additional oxygen ion implantation sothat the peak position of the distribution of implanted oxygen islocated at a portion lower than the interface between the buried oxidelayer, already formed, and the substrate thereunder; and then applyinganother high temperature heat treatment.
 2. A method of producing aSIMOX substrate wherein a buried oxide layer and a surface silicon layerare formed by applying the implantation of oxygen ions in a siliconsubstrate and a high temperature heat treatment thereafter,characterized by: forming the buried oxide layer through applying a hightemperature heat treatment after an oxygen ion implantation; thenapplying an additional oxygen ion implantation so that the dosing amountof the additional oxygen ion implantation does not exceed the totaldosing amount of the oxygen ions implanted earlier and that the peakposition of the distribution of implanted oxygen is located at a portionlower than the interface between the buried oxide layer, already formed,and the substrate thereunder; and then applying another high temperatureheat treatment.
 3. A method of producing a SIMOX substrate wherein aburied oxide layer and a surface silicon layer are formed by applyingthe implantation of oxygen ions in a silicon substrate and a hightemperature heat treatment thereafter, characterized by: forming theburied oxide layer through applying a high temperature heat treatmentafter an oxygen ion implantation; then applying an additional oxygen ionimplantation so that the accelerating energy formerly applied to theoxygen ion implantation is different from that applied to the additionaloxygen ion implantation and that the peak position of the distributionof implanted oxygen is located at a portion lower than the interfacebetween the buried oxide layer, already formed, and the substratethereunder; and then applying another high temperature heat treatment.4. A method of producing a SIMOX substrate wherein a buried oxide layerand a surface silicon layer are formed by applying the implantation ofoxygen ions in a silicon substrate and a high temperature heat treatmentthereafter, characterized by: forming the buried oxide layer throughapplying a high temperature heat treatment after an oxygen ionimplantation; then applying an additional oxygen ion implantation sothat the dosing amount of the additional oxygen ion implantation doesnot exceed the total dosing amount of the oxygen ions implanted earlier,that the accelerating energy formerly applied to the oxygen ionimplantation is different from that applied to the additional oxygen ionimplantation and that the peak position of the distribution of implantedoxygen is located at a portion lower than the interface between theburied oxide layer, already formed, and the substrate thereunder; andthen applying another high temperature heat treatment.
 5. A method ofproducing a SIMOX substrate wherein a buried oxide layer and a surfacesilicon layer are formed by applying the implantation of oxygen ions ina silicon substrate and a high temperature heat treatment thereafter,characterized by: forming the buried oxide layer through applying a hightemperature heat treatment after an oxygen ion implantation; thenpartially removing the surface of the surface silicon layer alreadyformed; then applying an additional oxygen ion implantation so that thepeak position of the distribution of implanted oxygen is located at aportion lower than the interface between the buried oxide layer, alreadyformed, and the substrate thereunder; and then applying another hightemperature heat treatment.
 6. A method of producing a SIMOX substratewherein a buried oxide layer and a surface silicon layer are formed byapplying the implantation of oxygen ions in a silicon substrate and ahigh temperature heat treatment thereafter, characterized by: formingthe buried oxide layer through applying a high temperature heattreatment after an oxygen ion implantation; then partially removing thesurface of the surface silicon layer already formed; then applying anadditional oxygen ion implantation so that the dosing amount of theadditional oxygen ion implantation does not exceed the total dosingamount of the oxygen ions implanted earlier and that the peak positionof the distribution of implanted oxygen is located at a portion lowerthan the interface between the buried oxide layer, already formed, andthe substrate thereunder; and then applying another high temperatureheat treatment.
 7. A method of producing a SIMOX substrate wherein aburied oxide layer and a surface silicon layer are formed by applyingthe implantation of oxygen ions in a silicon substrate and a hightemperature heat treatment thereafter, characterized by: forming theburied oxide layer through applying a high temperature heat treatmentafter an oxygen ion implantation; then partially removing the surface ofthe surface silicon layer already formed; then applying an additionaloxygen ion implantation so that the accelerating energy formerly appliedto the oxygen ion implantation is different from that applied to theadditional oxygen ion implantation and that the peak position of thedistribution of implanted oxygen is located at a portion lower than theinterface between the buried oxide layer, already formed, and thesubstrate thereunder; and then applying another high temperature heattreatment.
 8. A method of producing a SIMOX substrate wherein a buriedoxide layer and a surface silicon layer are formed by applying theimplantation of oxygen ions in a silicon substrate and a hightemperature heat treatment thereafter, characterized by: forming theburied oxide layer through applying a high temperature heat treatmentafter an oxygen ion implantation; then partially removing the surface ofthe surface silicon layer already formed; then applying an additionaloxygen ion implantation so that the dosing amount of the additionaloxygen ion implantation does not exceed the total dosing amount of theoxygen ions implanted earlier, that the accelerating energy formerlyapplied to the oxygen ion implantation is different from that applied tothe additional oxygen ion implantation and that the peak position of thedistribution of implanted oxygen is located at a portion lower than theinterface between the buried oxide layer, already formed, and thesubstrate thereunder; and then applying another high temperature heattreatment.
 9. A method of producing a SIMOX substrate according to claim5, characterized in that the method of removing the surface of thesurface silicon layer is any one of etching using a reactive substance,the removal of an oxide layer after forming it by oxidizing the siliconat the surface of the substrate, and surface polishing.
 10. A method ofproducing a SIMOX substrate according to claim 6, characterized in thatthe method of removing the surface of the surface silicon layer is anyone of etching using a reactive substance, the removal of an oxide layerafter forming it by oxidizing the silicon at the surface of thesubstrate, and surface polishing.
 11. A method of producing a SIMOXsubstrate according to claim 7, characterized in that the method ofremoving the surface of the surface silicon layer is any one of etchingusing a reactive substance, the removal of an oxide layer after formingit by oxidizing the silicon at the surface of the substrate, and surfacepolishing.
 12. A method of producing a SIMOX substrate according toclaim 8, characterized in that the method of removing the surface of thesurface silicon layer is any one of etching using a reactive substance,the removal of an oxide layer after forming it by oxidizing the siliconat the surface of the substrate, and surface polishing.
 13. A SIMOXsubstrate produced by the production method according to any one ofclaims 1 to 12, characterized in that the thickness of the surfacesilicon layer of the SIMOX substrate is 10 to 400 nm, and the thicknessof the buried oxide layer thereof is 60 to 250 nm.